The ro-vibrational analysis of the <i>v</i>₄ fundamental band of CF₃Br from jet-cooled diode laser and FTIR spectra in the 8.3-μm region

The <i>v</i>₄ fundamental band of CF₃⁷⁹Br and CF₃⁸¹Br, present in natural isotopic abundance, was investigated in the 8.3-μm region by high-resolution infrared spectroscopic techniques. Tuneable diode laser spectra were recorded in the ranges 1202.5–1205.0 cm⁻¹, 1208.0–1210.1 cm⁻¹ and 1212.5–1214.5 cm⁻¹. The tuneable diode laser spectra were obtained at the reduced temperature of 200 K and in a free-jet expansion. The latter technique was used to reduce spectral congestion, achieving a rotational temperature of about 50 K, with a resolution up to 0.0008 cm⁻¹. A Fourier transform infrared spectrum covering the entire spectral region of the <i>v</i>₄ band, between 1190 and 1220 cm⁻¹, was recorded at 298 K with a resolution of 0.004 cm⁻¹. The experimental wavenumbers from the different spectroscopic techniques were combined to accomplish the complete ro-vibrational analysis of <i>v</i>₄. In total, 4651 transitions were assigned to CF₃⁷⁹Br, 4047 to CF₃⁸¹Br, with <i>J</i>″_max = <i>K</i>″_max=80; of these, 3171 for CF₃⁷⁹Br and 2755 for CF₃⁸¹Br are from diode laser measurements. The data of each isotopologue were analysed using the model Hamiltonian for a degenerate vibrational state of a molecule of <i>C</i>₃_<i>v</i> symmetry. The <i>v</i>₄ band of both the isotopologues resulted essentially unperturbed, but the Δ<i>l</i> = Δ<i>k</i> = ±2 <i>l</i>-resonance was found to be active within the <i>v</i>₄ = 1 state. Precise values of the vibrational energy and of the ro-vibrational parameters of <i>v</i>₄ = 1 for CF₃⁷⁹Br and CF₃⁸¹Br were obtained. The bromine isotopic splitting amounts to 6.9 × 10⁻³ cm⁻¹. In addition, the equilibrium geometry and the harmonic force field were calculated <i>ab initio</i> using the large-size basis set def2-QZVP in conjunction to the PBE0 functional.

本研究针对天然同位素丰度下的三氟甲基溴-79（CF₃⁷⁹Br）与三氟甲基溴-81（CF₃⁸¹Br）的v₄基频带，于8.3 μm光谱区域采用高分辨红外光谱技术开展了系统研究。可调谐二极管激光（Tuneable diode laser）光谱的采集范围涵盖1202.5–1205.0 cm⁻¹、1208.0–1210.1 cm⁻¹以及1212.5–1214.5 cm⁻¹三个区间，实验在200 K的降温条件下结合自由射流膨胀技术完成。该技术可有效降低光谱拥挤效应，将转动温度控制在约50 K，光谱分辨率可达0.0008 cm⁻¹。本研究同时采集了覆盖v₄带全光谱区间（1190–1220 cm⁻¹）的傅里叶变换红外（Fourier transform infrared, FTIR）光谱，实验温度为298 K，光谱分辨率为0.004 cm⁻¹。研究将不同光谱技术获得的实验波数进行整合，完成了v₄带的完整转动振动（ro-vibrational）分析。其中，CF₃⁷⁹Br共归属4651条跃迁，CF₃⁸¹Br共归属4047条跃迁，二者的最高转动量子数与简并量子数均满足J''ₘₐₓ = K''ₘₐₓ = 80；上述跃迁中，分别有3171条CF₃⁷⁹Br与2755条CF₃⁸¹Br的跃迁数据来自可调谐二极管激光测量。针对每一种同位素异构体（isotopologue），本研究采用C₃ᵥ对称分子简并振动态的哈密顿模型进行分析。结果表明，两种同位素异构体的v₄带均基本未受扰动，但在v₄=1振动能级中，Δl=Δk=±2的l共振效应依然活跃。本研究得到了CF₃⁷⁹Br与CF₃⁸¹Br的v₄=1振动能级及转动振动参数的精确数值，溴同位素分裂量为6.9×10⁻³ cm⁻¹。此外，本研究结合def2-QZVP大尺寸基组与PBE0泛函，通过从头算（ab initio）方法计算了该分子的平衡几何构型与简谐力场。